1. Field of the Invention
The present invention relates to a color image reading apparatus for reading a color image.
2. Related Background Art
Conventional image sensors are classified into silicon crystal type sensors (e.g., CCD's and bipolar sensors) and thin-film sensors (e.g., Cds and amorphous silicon sensors). These image sensors can also be classified .into reduction and one-to-one sensors in view of their differing optical arrangements. Color separation schemes of conventional color image reading apparatuses are classified into schemes for switching light sources or color filters using a single image sensor, and schemes without switching, i.e., simultaneous reading and color separation.
Typical simultaneous reading and color separation schemes are a scheme for time-divisionally reading out color separation signals through stripe filters constituting a one-line image sensor in accordance with a dot sequence, and a scheme for reading out color separation signals by a plurality of parallel image sensors in units of separation colors in accordance with a line sequence. A thin-film image reading apparatus is suitable for a high-speed image reading apparatus due to a high reading speed, and a one-to-one image reading apparatus is suitable for a high-sensitivity image reading apparatus due to a wide light receiving area if the read resolution remains the same.
A high-sensitivity image reading apparatus is required especially for a color image reading apparatus in view of a decrease in incident light quantity due to the color separation filters, and the spectral sensitivity characteristics of the image sensor itself. In order to realize high-speed reading by using a light source having a light quantity within a practical range, stripe filters are used for a one-to-one silicon crystal image sensor. However, it is very difficult to manufacture a silicon crystal sensor chip which can cover the length (i.e., 297 mm) of an A4 size due to manufacturing limitations. In recent years, as disclosed in U.S. Pat. No. 4,891,690, assigned in common with the present invention, a plurality of image sensors are arranged in a specific layout to constitute a one-line sensor as a high-speed sensor.
When a plurality of image sensors are connected in a main scanning direction, e.g., a color separation stripe filter of blue (B), green (G), and red (R) is arranged with a read resolution of 16 dots/mm, an interval between the adjacent pixels is 20.8 .mu.m (=1/16 mm .times..DELTA.). With the above arrangement, it is possible to perform alignment between the respective image sensors, and a high resolution can be obtained without encountering problems concerning positional precision of a read image. However, read density errors between image sensor chips by nonuniformity of the respective image sensors, and particularly color registration errors between image sensor channels in the case of a color reading apparatus pose a serious problem. The causes for density and color registration errors between the image sensor chips are (1) variations in sensitivity and dark current outputs between the image sensor chips, (2) variations in characteristics of chips or signal processors for the respective filters, and the like. As disclosed in U.S. Pat. No. 4,558,357, assigned in common with the present invention, a plurality of image sensors are arranged in a subscanning direction, and R, G, and B color separation filters are coated thereon to provide a plurality of line sensors. By using these line sensors, density and color registration errors between the image sensor chips can be reduced.
In such a reading apparatus, however, sensitivity levels are different in units of colors. The light quantity of a light source is set on the basis of a light receiving element having the lowest sensitivity level. A sensitivity correction filter is inserted for an element of a color having a high sensitivity level, thereby normalizing output levels.
In order to normalize the output levels, a gain of an amplifier for amplifying an output from an element of a color having a low sensitivity level is increased, and a gain of an amplifier for amplifying an output from an element of a color having a high sensitivity level is reduced.
With the arrangement wherein the light quantity of the light source is determined on the basis of the low-sensitivity element, the light output of the light source is wasted for the element having a high sensitivity level, and power is also wasted. In addition, the power required for an original illumination lamp is increased, thus posing a problem concerning temperature rise in the apparatus.
With the arrangement wherein the gain of the amplifier is changed in accordance with output levels of elements, the output level of an element of a color having a low sensitivity level is low, and a high S/N ratio cannot be obtained. As a result, a high-quality signal cannot be obtained.
In a system including a plurality of parallel image sensors to read out color separation signals in accordance with a line sequence or in a system including a silicon crystal stripe filter to read out color separation signals in accordance with a dot sequence, output ratios of the respective filters are greatly different from each other according to total spectral sensitivity characteristics (FIG. 4) of a color reading unit due to the following reasons:
(1) Decreases in incident light quantities by color separation filters, and especially, imbalance of transmittances of the respective color separation filters; and
(2) There is no light source having a smooth spectral energy distribution in a necessary wavelength region.
For example, when a color CCD having a spectral sensitivity distribution shown in FIG. 2 is combined with a halogen lamp having a spectral distribution characteristics shown in FIG. 3, the output ratios of the filters are greatly different from each other, as described above. When signal processing shown in FIG. 1 is performed using these output values, S/N ratios are different from each other in units of colors. When color correction such as masking is finally performed, image quality is undesirably degraded.
In order to correct this, there may be proposed a scheme wherein a specific light source such as a bluish white color (BW) fluorescent tube having a high output level at shorter wavelengths and a low output level at longer wavelengths is used to obtain a relative output ratio of 1:1:1, as shown in FIG. 6. However, the spectral distribution characteristics of the light source must be redesigned according to the specifications of the sensor, thus resulting in complex design and high cost. FIG. 7 shows spectral distribution characteristics of th bluish white (BW) fluorescent tube.
Referring to FIG. 1, a 3-line full-color line sensor 100 includes a B-CCD 101 having a blue (B) filter, a G-CCD 102 having a green (G) filter, and an R-CCD 103 having a red (R) filter. These filters are formed on a single wafer. Signal processors 701, 702, and 703 amplify output signals from the CCDs 101, 102, and 103 to voltage levels corresponding to reference levels of A/D converters 704, 705, and 706, respectively. The signal processors 702 and 703 have the same arrangement as that of the signal processor 701, and a detailed illustration thereof is omitted.
Referring to FIG. 1, the signal processor 701 includes a first amplifier 701a, a second amplifier 701b, an S/H circuit 701c, and a comparator 701d. The S/H circuit 701c samples and holds an output level of a dark output signal section which shields the light receiving section with aluminum and receives a dark current component of output signals from the CCD shown in FIG. 5. The comparator 701d compares the dark output signal level with the reference level. The second amplifier 701b, the S/H circuit 701c, and the comparator 701d constitute a feedback clamp circuit for maintaining the reference level of an amplified output signal from each CCD constant. The first and second amplifiers 701a and 701b amplify each CCD output signal to a dynamic range of the A/D converter 704. The output signal ratios of the respective B-CCD 101, G-CCD 102, and R-CCD 103 are different from each other. If the output values of the B-CCD 101, the G-CCD 102, and R-CCD 103 are given as 200 mV, 400 mV, and 600 mV, and the dynamic range of each A/D converter 704, 705, or 706 is given as 2 V, the total gains of the respective signal processors are given as 10 (=2 V/200 mV) for B, 5 (=2 V/400 mV) for G, and 3.3 (.apprxeq.2 V/600 mV) for R. Since noise levels from the respective CCD are equal to each other, noise levels of the B and G components with respect to the R signal are given as 3 (.apprxeq.10/3.3) for B and 2 (=10/5) for G. In this manner, the S/N ratios are relatively decreased.